Titanium and its alloys have unique combinations of low densities and high melting points which lead to their widespread application in high technology applications, particularly in gas turbine engines.
A large number of titanium alloys have been proposed. These generally comprise at least about 80% titanium with balance being other additions including aluminum, vanadium, chromium, zirconium, etc. The widely used commercial alloys of this type have either an alpha or beta structure, both of which are essentially titanium solid solutions.
Research has also been conducted aimed at utilizing various titanium intermetallic compounds based on titanium. These include Al.sub.3 Ti, Ti.sub.3 Al, and TiAl. The TiAl composition is the one of interest with respect to this invention. It has a high melting point, approximately 2600.degree. F., and a low density, even lower of that of titanium because of the large amount of aluminum present. One draw back of TiAl is its lack of useful ductility. Various alloying approaches have been taken to overcome this problem with a certain degree of success. U.S. Pat. No. 4,294,615 which shares a common assignee with the present application, discloses that the addition of a small amount of vanadium increases the ductility of TiAl type compositions and that the addition of a small amount of carbon increases the creep rupture strength of such materials. This patent also describes some of the earlier work in the TiAl system. The patent is incorporated herein by reference.
Being an intermetallic material with high strength, low to moderate ductility, and high melting point, TiAl type alloys have in the past been formable only with the greatest of difficulty. Invariably, forming is conducted at a high temperature, generally in excess of about 2400.degree. F. for reasons of ductility. This requirement poses a problem for the production of certain thin section alloy shapes, particularly sheet material. Sheet material is formed by rolling, but when thin sheet is being formed, the heat extraction capability of the rolls is such that the material between the rolls rapidly loses its heat and then cracks as it becomes too cold. The obvious approach would be to heat the rolls to the hot rolling temperature, but this is impractical given the temperatures involved. To my knowledge crack free TiAl sheet having a thickness of 0.1 inch and below has never been produced.
Similar difficulties can be envisioned in forming TiAl type materials in different thin section shapes by other processes such as by forging.
Other titanium-aluminum compounds do not suffer from this great lack of ductility. In particular, Al.sub.3 Ti and Ti.sub.3 Al display useful ductilities.
As used herein, the terms Ti.sub.3 Al, TiAl.sub.3 and TiAl include minor alloying elements which do not significantly change the crystal structure of the phases. These terms are also intended to denote materials which contain up to about 10% by volume of other phases in the case of Ti.sub.3 Al and Al.sub.3 Ti and up to about 20% by volume of other phases in the case of TiAl. That is to say, a structure comprised of 85 volume % TiAl, 5 volume % Al.sub.3 Ti and 10 volume % Ti.sub.3 Al is considered to be TiAl.